The invention relates to a method of determining the average effect of radio wave multipath fading in a desired area in a radio system, said method using an at least two-dimensional digital map to describe the desired area of a covering area of a base station in the system, and said method determining the average effect of multipath fading of a transmitter output in different sub-areas of the desired area.
When a radio system is being constructed an attempt is made to achieve a desired coverage area with minimum costs. When considering the locations of the base stations of the system, the required traffic capacity and the achieved coverage area are taken into account. The aim is to locate the base stations so as to ensure an extensive coverage area and an advantageous location of the base station as far as the radio wave propagation is concerned.
Different methods and instruments have been developed for radio network planning. Digital maps providing modelled information on the terrain and buildings in the desired area are commonly used instruments in radio network planning. By means of a digital map a computer can be used to calculate coverage areas and parameters concerning the network operation for different base station locations.
When determining the base station coverage area it is important to determine signal fading in different areas. As the system behaviour is simulated signal fading also has to be modelled as realistically as possible. Particularly in an urban environment fading is a variable changing in time depending on the geometry of the buildings, walls and other scattering surfaces surrounding the base station and the terminal.
Fading is conventionally divided into two different types, fast and slow fading, but this is a very rough division. In reality signal multipath fading is caused by phase differences, and as the terminal moves the phase difference change results in back and forth variation of the signal strength typically seen at correlation distances from half a wavelength to hundreds of wavelengths depending on the environment.
In prior art solutions, in moving radio system simulators, for example in link-level simulators implemented by COSSAP, fadings are simulated by adding attenuation and fading to the transmitted signal. Fast fading is typically generated by simulating a stationary random process according to Rayleigh or Rice distributions. Hereafter fading is averaged using an appropriate filter.
In network planning software using the digital map, the signal attenuation is calculated, for example, on the basis of an empirical model where multipath fading is not taken into account except for certain special cases.
However, prior art methods have several problems and defaults. In typical link-level simulators implemented, for example by COSSAP, the receiving algorithms are realistic, but the problem is whether the channel model is realistic or not. A realistic channel model can be attained by ray tracing i.e. a ray search channel model. This is, however, obstructed by the computational complexity required by a ray tracing method impeding the implementation of the method.
In network planning software the resolution of the digital map can be raised to increase accuracy but then fading should be taken into account in order to calculate the strength of the field by the map resolution. In this case too, the ray tracing model taking multipath fading into account would improve accuracy but the required computation amount is too high.
An object of the present invention is to provide a method by which the average effect of fading can be calculated with acceptable accuracy without a high computation load.
This is achieved by the method of the type set forth in the preamble characterized in that signal fading parameters are determined at some point in each sub-area by a ray tracing method and that the average effect of signal multipath fading in a sub-area is calculated using said parameters.
The method of the invention has several advantages. In the solution of the invention the computation time is short, a fraction of the time taken if the entire calculation were performed using the ray tracing method, in which case calculating an average for the sub-areas would require several computation points for each sub-area. The ray tracing method is currently the most accurate known method for obtaining realistic results of fading and attenuation values. In the solution of the invention, when the size of the sub-areas is appropriately selected, an estimation result that is acceptably accurate is, however, obtained.